The present invention relates generally to the field of photolithography and, more particularly, to methods for using photolithographic techniques for making optical structures such as lenses that are deeper than 100 micron.
Photolithography is a technique that is well known for making semiconductors, microlenses, diffraction gratings, diffractive lenses, microelectronic structures etc. The outstanding characteristic of photolithography is that it is capable of producing complex structures with accuracy in the nanometer range.
Typically, in photolithography, a coating of photoresist is applied to a silicon or fused silica substrate such as a wafer. A laser is then used to write the desired structure into the photoresist from above the photoresist. By focusing the laser to a very small spot and moving the laser in a precisely controlled manner, extremely precise structures can be created in the photoresist. After the pattern has been written onto the photoresist, a chemical wash is used to remove the photoresist preferentially between the areas that have been exposed and those that have not been exposed, thereby leaving the structure as written.
The limitation with photolithography is in the depth of the structures that can be created. This limitation comes from a combination of the laser power, the transparency of the photoresist to the laser light, and the thickness of the photoresist layer. While laser power can be increased to create a deeper structure, and alternate photoresists or other wavelength lasers could be used to increase the effective transparency of the photoresist; the thickness of the photoresist can only be increased a limited amount.
Photoresist is a liquid material as it is applied to the wafer. In order to obtain a uniform thickness of the photoresist, a spin coating technique is typically used to spread the liquid photoresist over the substrate surface. For the final structure to be flat and of uniform depth, the photoresist must be applied very uniformly over the wafer surface. The spin coating technique is very good at creating a uniform coating in the range of 1-500 micron depending on the spin speed and the type of photoresist used. For many photoresists, the thickness range that is possible with spin coating is very narrow, for example the 3612 photoresist from Shipley (Marlborough, Mass.) is only capable of thicknesses of 1-1.6 micron and the SPR220 photoresist from Shipley is capable of 7-18 micron. At the other extreme is the SU8 photoresist from Microchem Corp (Newton, Mass.) that is capable of thicknesses of 2-500 micron on a spin coater. Above the maximum thicknesses listed, the photoresist flows off the edge of the wafer during spin coating.
In U.S. Pat. No. 4,340,654, Campi describes a process for repairing photomasks which have clear defects. The process uses a laser applied to a photomask on a substrate so that in areas where there is a hole in the photomask, the laser penetrates the photomask to an opaque powdered material which has been applied on the other side of the photomask. The laser then melts the powdered material only in the area of the defect, thereby fusing the melted material to the photomask and repairing the defect. The process, as described by Campi, is specifically directed at opaque photomasks using a thermally based process to fuse an opaque material that absorbs the radiant energy. In a further embodiment, Campi does present a variation on photomask repair and instead describes a similar technique for producing photomasks directly. In this case, Campi is focused on fusing opaque materials to make photomasks without requiring the use of the usual prior art techniques of first coating the substrate surface with a photoresist material. Consequently, the method presented by Campi is not applicable to making optical structures which by necessity are made of transparent photoresist materials and which are three dimensional with significant depth.
Van Dine also describes the use of a laser which is directed through a transparent substrate in U.S. Pat. No. 4,705,698. In this case however, the laser is used to scribe the substrate through ablation to create a separation between semiconductor layers. This approach is very different from using the laser to create an optical surface in photoresist.
Tankovich, in U.S. Pat. No. 5,614,339, also discloses the use of a laser through a transparent substrate to remove opaque printing through ablation on the opposite side for the purpose of enabling toner materials to be recycled. Again the laser is used in an approach that is only suitable for use with opaque materials.
In a variation on Tankovich, Chrisey, in U.S. Pat. No. 6,177,151, discloses a similar ablation approach with a laser transmitted through a transparent substrate to an opaque layer which ablates. However, in Chrisey, the ablated material is captured onto a receiving substrate, thereby creating a method for printing with the ablated material.
The prior art fails to teach a method suitable for making three-dimensional optical structures using photoresist. In addition, the thermal and ablation aspects disclosed in the prior art all involve significant transfer of energy from the laser in that the material either melts or is ablated. In creating an optical structure, the transfer of energy from the laser must be minimal to protect the optical properties of the photoresist material being processed. In addition, the prior art does not disclose methods suitable for building structures of significant depth.
It is an object of the present invention to provide a method for producing optical structures of a photoresist material which have a depth that is greater than 100 micron.
It is a further object of the present invention to disclose a method for making optical structures in photoresist at substantially greater thicknesses than can be provided by spin coated coatings.
It is a further object of the present invention to reduce the cost of making photoresist structures by eliminating the multiple spin coating steps needed to produce thick photoresist coatings for thick photoresist structures.
Briefly stated, these and numerous other features, objects and advantages of the present invention will become readily apparent upon a reading of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by a method utilizing a pool of photoresist on top of a transparent substrate such that the laser can be located under the substrate. Optical structures can be created in the photoresist by moving the laser in a pattern that matches the desired optical structure and the laser beam is transmitted through the substrate and into the pool of photoresist.
By using a pool of photoresist on top of the substrate, the application of the photoresist is greatly simplified compared to spin coating in that it can be merely poured onto the substrate. If very thick layers of photoresist are desired, cylindrical walls can be attached to the edges of the substrate to create a container which can act to contain the liquid photoresist during processing. In this case, the thickness of the photoresist is limited only by the height of the walls of the container. In either case, the processing cost of the present invention in applying the photoresist is substantially less than spin coating since the process is much simpler.
By transmitting the laser through the substrate, the flatness of the base of the optical structure produced is determined by the flatness of the substrate rather than the flatness of the surface of the coated photoresist as in the prior art. Due to the extreme requirements of the semiconductor industry, substrates in the form of wafers of various materials can be easily obtained which are very flat. Alternately, the invention as described could be used with substrates that are intentionally contoured to create a photoresist structure on top of a contoured substrate such as in a micro device which has optical and mechanical or electrical features.
Thus, the present invention defines a technique to replace spin coating which will enable photoresists of all types to be effectively used at an increased thickness so that deeper optical structures can be made with uniform quality. The invention involves using wafers that are transparent to the laser that is used for writing onto the photoresist. The laser can then be located under the wafer and write up through the wafer onto the photoresist. By writing through the wafer, the photoresist can then be poured onto the wafer in any thickness and coating uniformity is no longer an issue since structure depth is determined by the laser writing process and flatness is determined by the wafer. The invention also enables optical structures to be created in photoresist on contoured substrates. The term xe2x80x9coptical structuresxe2x80x9d as used herein is intended to include, for example, lenses, gratings, and arbitrary diffractive surfaces.